Many display devices, such as laser printers and ink-jet printers, form their displays in a raster-scan manner: the ink dispenser or laser beam scans from left to right and/or right to left in successive rows on the display medium. In this way, it renders an image that at some point has been represented in a raster-scan format as discrete picture-element ("pixel") values. This representation, as well as processing employed to command the display device to render it, are based on the assumption of a regular pixel spacing: the row-to-row spacing is assumed to be uniform. But it is difficult to cause reality to match this ideal so closely that imperfections are not detectable, so one must either exercise great care and suffer the attendant expense in order to achieve the required uniformity or tolerate the artifacts that result from failure to do so.
One of the undesirable visual artifacts of this lack of uniformity is a type of banding that is sometimes seen in regions where the display device is attempting to render a uniform gray or other color level. Instead of the uniform color, the non-uniform scan-line spacing ends up causing bands of lighter and darker regions. This typically results from interaction of the non-uniform line spacing with the half-toning process that the printer or similar display device uses to render shades of gray. At any given pixel, devices such as ink-jet and laser printers are typically capable only of on-and off operation: they either deposit a dot of the imaging agent in the pixel or not. In a monochrome printer, for instance, a given pixel is only black or white; there is no shade of gray.
In order to render gray-scale values, printers rely instead on duty cycle. In regions that are intended to be darker, more pixels receive dots. In lighter regions, fewer do. One way of achieving this result is to have groups of dots form clusters, which are larger or smaller in accordance with the intended gray level to be rendered. The effect is quite similar to that of the traditional photoengraving half-tone screen. Non-uniform spacing can make clusters formed by different row sequences differ in size and thereby cause unintended variations in displayed color. These variations tend to form undesirable visible bands.
One can avoid this effect by employing different half-toning techniques, such as distributed-dot dithering, in which the "turned-on" pixels are not clustered together. Because clusters generally do not form, non-uniform scan-line spacing does not change cluster size, so the non-uniform line spacing is less evident. But this method relies to a certain extent on the display device's ability to display isolated pixels, and certain types of display devices are limited in this respect.